winbindd fails with "Got sig[15] terminate (is_parent=1)"

`/var/log/samba/log.winbindd` reveals

    [2016/02/07 19:03:53, 0] ../source3/winbindd/winbindd.c:1490(main)
      winbindd version 4.1.17-Ubuntu started.
      Copyright Andrew Tridgell and the Samba Team 1992-2013
    [2016/02/07 19:03:53.123783, 0] ../source3/winbindd/winbindd_cache.c:3196(initialize_winbindd_cache)
      initialize_winbindd_cache: clearing cache and re-creating with version number 2
    [2016/02/07 19:03:53.126663, 0] ../lib/util/become_daemon.c:136(daemon_ready)
      STATUS=daemon 'winbindd' finished starting up and ready to serve connectionsGot sig[15] terminate (is_parent=1)

every time I trigger a restart with `sudo systemctl restart winbind.service`.
---
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
DistroRelease: Ubuntu 15.10
InstallationDate: Installed on 2015-04-20 (293 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
NmbdLog:

NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
Package: samba 2:4.1.17+dfsg-4ubuntu3.1
PackageArchitecture: amd64
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SambaServerRegression: Yes
SmbConfIncluded: Yes
Tags: wily
Uname: Linux 4.3.3-040303-generic x86_64
UpgradeStatus: Upgraded to wily on 2015-10-26 (104 days ago)
UserGroups: adm cdrom dip docker libvirtd lpadmin lxd plugdev sambashare sudo wireshark
WindowsFailedConnect: Yes
_MarkForUpload: True
---
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
DistroRelease: Ubuntu 16.04
InstallationDate: Installed on 2015-04-20 (387 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
NmbdLog:

Package: samba 2:4.3.9+dfsg-0ubuntu0.16.04.1
PackageArchitecture: amd64
ProcEnviron:
 TERM=screen.xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SambaServerRegression: Yes
SmbConfIncluded: Yes
SmbLog: Failed to fetch record!
Tags: xenial
Uname: Linux 4.5.0-040500-generic x86_64
UpgradeStatus: Upgraded to xenial on 2016-03-03 (68 days ago)
UserGroups: adm cdrom dip docker fuse libvirtd lpadmin lxd plugdev sambashare sudo wireshark
WindowsFailedConnect: Yes
_MarkForUpload: True